744 Description of Land and Fresh-water Shells [Nor. 



met with to the westward. In some waters the species attains a great 

 size, being 0.9 of an inch in diameter, and nearly equalling in mag- 

 nitude PI. corneas, which it resembles in general appearance, but 

 from which it will be found to differ in the depth and regularity of 

 the striae, in the comparatively less profound excavation of the sutures 

 on the upper side, and less profoundly sunk apex, as well as in being 

 somewhat less ventricose. The animal is most commonly of a black 

 olive colour, occasionally dark maroon red. 



In the foregoing descriptions of Planorbis I have used the word* 

 upper and lower with reference to the faces of the disc which adjoin 

 respectively the back and foot of the animal when creeping. The 

 animal of Planorbis is undoubtedly sinistral, but if the shell be view- 

 ed as such practically, and placed with the side which would in a 

 sinistral shell be accounted the apex uppermost, it will be found 

 that the animal is on its back, and that it will have to twist its body 

 half round in order to gain the ground with its foot ; and that in order 

 to creep with any ease, it must reverse the position of its shell. This 

 will be more especially observable in the flatter and more oblique 

 mouthed species. I propose, therefore, to consider that face as con- 

 taining the apex, in discoid shells, which is contiguous to the back 

 of the animal. This side may invariably be known in Planorbis by 

 the greater projection of the lip in that part, by the deeper depression 

 of the central umbilicus, and by the more considerable involution of 

 the whorls occasioning a greater depth of suture. 



Observers have, in general, adhered to no fixed rule on the subject, 

 and have been guided chiefly by the aspect of the shell. Turton's 

 characters of PL fontanus and PL contortus afford an instance of the 

 same side being considered the upper in one species, and the lower 

 in another. Lamarck was more consistent, and while he rightly view- 

 ed the shell as sinistral, called, in every species, that side of the shell 

 which is contiguous to the back of the animal, the lower face. On 

 the other hand, it is evident, from Say's description of PL deflexus 

 and PL corpulentus, and from his denominating those species dextral, 

 that, in those shells at least, he has followed the contrary rule. 



23. Lymncea chlamys. Testa translucente, cornea aut castanea, 

 elongato-ovata ; spira gracili, breviore, acuminata ; anfractu ultimo 

 infra prsecipue ventricoso ; suturis parce depressis ; apertura infra 

 patente, basi leviter evasa. 



The Lymnsea in the Silhet collection is the chestnut- coloured variety. 

 The paler kind is met with in great perfection in Lehtdra jhil, near 

 Bandras, in company with Planorbis compressus, and another fine 

 Lymnsea which I designate as Lymncea Butta. The Silhet shell has an 



